NOTE: As their name suggests hints are not exactly Team Fortress 2 cheats, and are usually just simple (but very useful) tips on how you can unlock secret level, features, find hidden areas and such. Hopefully these will make playing Team Fortress 2 even more entertaining.

Dynasty: Win 20 games.
Flamethrower: Set five enemies on fire in 30 seconds.
Grey Matter: Get 25 headshots as a Sniper.
Hard to Kill: Get five kills in a row without dying.
Hardcore: Accumulate 1000 total kills.
Head of the Class: Play a complete round with every class.
Impenetrable Defense: Successfully defend Dustbowl without giving up a capture.
Impossible Defense: Successfully defend Gravel Pit without giving up a capture.
Lightning Offense: Win Well in 5 minutes or less.
Master of Disguise: Trick an opposing medic into healing you.
Nemesis: Get five revenge kills.
Powerhouse Offense: Win 2Fort with a shutout.
Relentless Offense: Win Hydro without giving up a capture.
Sentry Gunner: Accumulate 10 sentry gun kills with a single turret.
Team Doctor: Accumulate 25000 heal points as a Medic.
With Friends Like these...: Play in a game with 7 or more players from your friends list.
World Traveler: Play a complete game on 2Fort, Dustbowl, Granary, Gravel Pit, Hydro, and Well (CP).

Steam Achievements - Medic specific Achievements
Your achievements and stats can be viewed in Steam under: "Community -> My profile -> View all my games -> Team Fortress 2 (view stats)" These achievements can only be earned while playing the medic class, by getting a certain number of these achievements you unlock new medic weapons:

Autoclave: Assist in burning 5 enemies with a single ÜberCharge on a Pyro.
Autopsy Report: Provide an enemy with a freezecam shot of you taunting above their ragdoll.
Bedside Manner: Be healing a teammate as he achieves an achievement of his own.
Big Pharma: Assist a Heavy in killing 20 enemies, where neither of you die.
Blast Assist: Assist in exploding 5 enemies with a single ÜberCharge on a Soldier.
Blunt Trauma: Assist in punching 2 enemies with a single ÜberCharge on a Heavy.
Chief of Staff: Accumulate 1 million total heal points.
Consultation: Assist a fellow Medic in killing 5 enemies in a single life.
Doctor Assisted Homicide: Assist in killing 20 nemeses.
Does It Hurt When I Do This?: Kill 50 Scouts with your syringe gun.
Double Blind Trial: Deploy an ÜberCharge within 8 seconds of a nearby enemy Medic deploying his.
Family Practice: ÜberCharge ten of your Steam Community Friends.
First Do No Harm: Play a full round without killing any enemies, and score the highest on a team of 6 or more players.
FYI I am A Medic: Use your bonesaw to kill 5 Spies who have been calling for 'Medic!'.
Grand Rounds: Heal 200 teammates after they've called for 'Medic!'.
Group Health: Work with 2 other Medics to deploy 3 simultaneous ÜberCharges.
House Call: Join a game that one of your friends is in and then deploy an ÜberCharge on him.
Hypocritical Oath: Kill an enemy Spy that you have been healing.
Infernal Medicine: Extinguish 100 burning teammates.
Intern: Accumulate 7000 heal points in a single life.
Medic Milestone 1: Achieve 10 of the achievements in the Medic pack.
Medic Milestone 2: Achieve 16 of the achievements in the Medic pack.
Medic Milestone 3: Achieve 22 of the achievements in the Medic pack.
Medical Breakthrough: Assist in destroying 5 enemy Engineer buildings with a single ÜberCharge on a Demoman.
Medical Intervention: Save a falling teammate from dying on impact.
Midwife Crisis: Heal an Engineer as he repairs his Sentrygun while it's under enemy fire.
Peer Review: Kill 50 Medics with your bone saw.
Placebo Effect: Kill 5 enemies in a single life, while having your ÜberCharge ready, but undeployed.
Play Doctor: In a team with no Medics, be the first person to switch to Medic after a teammate calls for 'Medic!', and then heal 500 health.
Preventive Medicine: Block the enemy from capturing a control point with an ÜberCharged teammate.
Quadruple Bypass: Heal a teammate who's taking fire from 4 enemies at once.
Sawbones: Hit enemies with your bonesaw 5 times in a row without dying or missing.
Second Opinion: ÜberCharge 2 teammates at once.
Specialist: Accumulate 10000 heal points health in a single life.
Surgical Prep: Have an ÜberCharge ready before the Setup phase ends.
Trauma Queen: Deploy 3 ÜberCharges in less than 5 minutes, and assist in 5 kills during that time.
Triage: Deploy an ÜberCharge on a teammate less than a second before they're hit by a critical explosive.
Ubi concordia, ibi victoria: Assist in killing 5 enemies on an enemy control point, in a single life.
You'll Feel a Little Prick: Assist in killing 3 enemies with a single ÜberCharge on a Scout.

Steam Achievements - Pyro specific achievements
Your achievements and stats can be viewed in Steam under: "Community -> My profile -> View all my games -> Team Fortress 2 (view stats)" These achievements can only be earned while playing the pyro class, by getting a certain number of these achievements you unlock new pyro weapons:
Arsonist: Destroy 50 Engineer buildings.
Attention Getter: Ignite 100 enemies with the flare gun.
Baptism by Fire: Force 10 burning enemies to jump into water.
BarbeQueQ: Cause a dominated player to leave the server.
Burn Ward: Ignite 3 medics that are ready to deploy an Uber-charge.
Camp Fire: Kill 3 enemies in a row, all within the same area.
Clearcutter: Kill 6 people with your axe in one life.
Combined Fire: Use your shotgun to finish off 20 players you've ignited.
Controlled Burn: Ignite 50 enemies capturing one of your control points.
Cooking the Books: Ignite 5 enemies carrying your intelligence.
Dance Dance Immolation: Kill 3 enemies while they're taunting.
Dead Heat: Kill an enemy in the same second that he kills you.
Fire and Forget: Kill 15 players while you're dead.
Fire Chief: Kill 1000 enemies
Firefighter: Kill 500 enemies.
Firewall: Ignite 5 Spies who have a sapper on a friendly building.
Firewatch: Ignite 10 snipers while they are zoomed in.
Freezer Burn: Provide enemies with freezecam shots of each of your taunts.
Got A Light?: Ignite an enemy Spy while he's flicking a cigarette.
Hot on Your Heels: Kill 50 enemies with your flamethrower, from behind.
Hot Potato: Reflect 100 projectiles with your compressed air blast.
Hotshot: Kill a Soldier with a reflected critical rocket.
I Fry: Ignite 10 disguised Spies.
Lumberjack: Kill 3 people with your axe in one life.
Makin' Bacon: Kill 50 Heavies with your flamethrower.
Next of Kindling: Ignite an enemy, and the Medic healing him.
OMGWTFBBQ: Kill an enemy with a taunt.
Pilot Light: Ignite a rocket-jumping Soldier while he's in midair.
Plan B: Kill 10 enemies while you're both underwater.
Pyromancer: Do 1 million points of total fire damage.
Pyrotechnics: Kill 3 enemies in a single uber-charge.
Second Degree Burn: Kill a burning enemy who was ignited by another Pyro.
Spontaneous Combustion: Ignite 10 cloaked Spies.
Trailblazer: Ignite 10 enemies that have recently used a teleporter.
Weenie Roast: Have 2 enemy Scouts on fire at the same time.

Steam Achievements - Scout Specific Achievements
Your achievements and stats can be viewed in Steam under: "Community -> My profile -> View all my games -> Team Fortress 2 (view stats)" These achievements can only be done as the Scout class. Doing 10 of them unlocks a new primary weapon, doing 16 unlocks a new secondary weapon, doing 22 of them unlocks a new melee weapon.
A Year to Remember: Get 2004 lifetime kills.
Artful Dodger: Dodge 1000 damage in a single life using your Bonk! Atomic Punch.
Batter Up: Perform 1000 double jumps.
Batting the Doctor: Kill a Medic that is ready to deploy an ÜberCharge.
Beanball: Stun a scout with their own ball.
Belittled Beleaguer: Kill an opposing player that has your intelligence while holding theirs.
Block the Plate: Block 50 point captures.
Brushback: Stun 50 enemies while they are capturing a point or pushing the cart.
Caught Napping: Kill 50 enemies from behind with the Force-A-Nature.
Closer: Destroy 3 teleporter entrances.
Doctoring the Ball: Kill 3 enemies while under the effects of a Medic's ÜberCharge.
Dodgers 1, Giants 0: Kill an enemy Heavy and take his Sandvich.
Fall Classic: Cause an environmental death or suicide using the Force-A-Nature's knockback.
First Blood: Get the first kill in an Arena match.
First Blood, Part 2: Kill 5 enemies with the First Blood crit buff.
Foul Territory: Cause an environmental death by stunning an enemy.
Gun Down: Destroy an active sentry gun using your pistol.
I'm Bat Man: Survive 500 damage in one life.
If You Build It: Destroy 3 enemy buildings while they are still under construction.
Moon Shot: Stun an enemy for the maximum possible duration by hitting them with a long-range ball.
No Hitter: Steal and then capture the enemy intelligence without firing a shot.
Out of the Park: Bat an enemy 25 meters.
Pop Fly: Kill 20 players while double-jumping.
Quick Hook: Kill a player in Well before the round starts.
Race for the Pennant: Run 25 kilometers.
Retire the Runner: Kill a Scout while they are snared by the post-dodge slowdown effect.
Round Tripper: Capture the enemy intelligence 25 times.
Set the Table: Initiate 10 point captures that ultimately succeed.
Side Retired: Capture the last point in a CP map.
Stealing Home: Start capping a capture point within a second of it becoming available.
Strike Zone: Kill 50 enemies while they are stunned.
The Big Hurt: Stun 25 ÜberCharged enemies.
The Cycle: In a single life, kill an enemy while you are on the ground, in the air, and in the water.
Triple Play: Capture three capture points in a row in one life.
Triple Steal: Capture the enemy intelligence 3 times in a single CTF round.

Heavy Achievements - Heavy Update
These achievements can only be earned while playing the heavy class, by getting a certain number of these achievements you unlock new heavy weapons:

0wn the Means of Production: Remove 20 stickybombs by killing the Demomen who produced them.
Borsht Belt: Kill 10 Heavies with The K.G.B
Class Struggle: Work with a friendly Medic to kill an enemy Heavy & Medic pair.
Communist Mani-Fisto: Kill an enemy with a critical punch.
Crime and Punishment: Kill 10 enemies carrying your intelligence.
Crock Block: Survive a direct hit from a critical rocket.
Division of Labor: Kill 20 enemies with a Medic assisting you, where neither of you die.
Don't Touch Sandvich: Kill 50 Scouts using Natascha.
Factory Worker: Kill 20 enemies while being recharged by a dispenser.
Five Second Plan: Kill an enemy in the first 5 seconds after you exit a teleporter.
Gorky Parked: Kill 25 enemies while you're standing on a control point you own.
Heavy Industry: Fire $200,000 worth of minigun rounds in a single life.
Heavy Milestone 1: Achieve 10 of the achievements in the Heavy Pack
Heavy Milestone 2: Achieve 16 of the achievements in the Heavy Pack
Heavy Milestone 3: Achieve 22 of the achievements in the Heavy Pack
Icing on the Cake: Get 20 kills on players that you're dominating.
Iron Kurtain: Take 1000 points of damage in a single life.
Kollectivization: Get 1000 assists.
Konspicuous Konsumption: Eat 100 sandviches.
Krazy Ivan: Kill 100 enemies while both you and your victim are underwater.
Lenin a Hand: Help 5 teammates get revenge on their nemeses.
Marxman: Kill 10 enemies in mid-air with the minigun.
Party Loyalty: Kill 50 enemies within 3 seconds of them attacking your medic.
Permanent Revolution: Kill 5 enemies without spinning down your gun.
Photostroika: Provide an enemy with a freezecam shot of you taunting while invulnerable.
Purge: Kill 15 enemies capturing a control point you own.
Pushkin the Kart: Get 50 caps on payload maps.
Rasputin: In a single life, get shot, burned, bludgeoned, and receive explosive damage.
Rationing: Kill an enemy with your shotgun while you're out of minigun ammo.
Red Oktoberfest: Earn a domination for a Medic who;s healing you.
Redistribution of Health: Heal 1000 damage with med-kits in a single life.
Show Trial: Kill an enemy with a taunt.
Soviet Block: While invulnerable and on defense, block an invulnerable enemy Heavy's movement.
Soviet Union: Get 25 enemy kills where you either assist or are assisted by another Heavy.
Spyalectical Materialism: Kill or assist in killing 10 cloaked Spies.
Stalin the Kart: Block the enemy from moving the payload cart 25 times.
Supreme Soviet: Get UberCharged 50 times.
Vanguard Party: Be the first on your team to start capturing a control point in a round.

Sniper Achievements - Sniper Update
These achievements can only be earned while playing the Sniper class, by getting a certain number of these achievements you unlock new Sniper weapons:

Australian Rules: Dominate an enemy Sniper.
Be Efficient: Get 3 kills with the Sniper Rifle without missing a shot.
Be Polite: Provide an enemy with a freezecam shot of you doffing your hat.
Beaux And Arrows: Kill a Heavy & Medic pair with the bow.
Consolation Prize: Get backstabbed 50 times.
De-sentry-lized: Destroy 3 Engineer sentry guns.
Dead Reckoning: Kill an enemy with an arrow while you're dead.
Dropped Dead: Kill a Scout in midair with your Sniper Rifle or the Huntsman.
Enemy At The Gate: Kill an opponent within the first second of a round.
Friendship Is Golden: Extinguish a burning teammate with your Jarate.
Have A Plan: Capture the flag in CTF.
Jarate Chop: Jarate and then kill 3 enemies with your Kukri.
Jarring Transition: Use Jarate to reveal a cloaked Spy.
Jumper Stumper: Kill a rocket or grenade-jumping enemy in midair with your Sniper Rifle or the Huntsman.
Kill Everyone You Meet: Kill 1000 enemies.
Kook The Spook: Kill 10 Spies with your Kukri.
My Brilliant Career: Top the scoreboard 10 times on teams of 6 or more players.
Not A Crazed Gunman, Dad: In a single life, kill 3 enemies while they are achieving an objective.
Number One Assistant: Score 5 assists with the Jarate in a single round.
Parting Shot: Headshot an enemy player the moment his invulnerability wears off.
Pincushion: Hit an enemy with 3 arrows, without killing them.
Rain On Their Parade: Jarate an enemy and the Medic healing him.
Robbin' Hood: Take down an intelligence carrier with a single arrow.
Rode Hard, Put Away Wet: Jarate an enemy that you're dominating.
Saturation Bombing: Jarate 4 enemy players with a single throw.
Self-destruct Sequence: Headshot 10 enemy Snipers.
Shafted: Stab an enemy with an arrow.
Shock Treatment: Kill a spy whose backstab attempt was blocked by your Razorback.
Shoot The Breeze: Kill a fully invisible Spy in a single hit.
Socket To Him: Headshot an enemy Demoman.
The Last Wave: Provide an enemy with a freezecam shot of you waving to them.
Triple Prey: In a single round, kill the same enemy with 3 different weapons.
Trust Your Feelings: Get 5 kills with the Sniper Rifle without your scope.
William Tell Overkill: Pin an enemy Heavy to the wall via his head.
Überectomy: Kill a Medic that is ready to deploy an ÜberCharge.

Spy Achievements - Spy Update
These achievements can only be earned while playing the spy class, by getting a certain number of these achievements you unlock new spy weapons:

A Cut Above: Kill a gun-wielding Spy with your knife.
Agent Provocateur: Backstab your Steam Community friends 10 times.
Burn Notice: Survive 30 seconds after being ignited while cloaked.
Come In From The Cold: Get a Revenge kill with a backstab.
Constructus Interruptus: Kill an Engineer who is working on a sentry gun.
Counter Espionage: Backstab a disguised Spy.
Deep Undercover: While using the Cloak and Dagger, kill the same enemy 3 times, all within the same area in a single life.
Die Another Way: Kill a Sniper after your backstab breaks his Razorback.
Diplomacy: Kill 50 enemies with the Ambassador.
Dr. Nooooo: Backstab a Medic that is ready to deploy an ÜberCharge.
For Your Eyes Only: Provide an enemy with a freezecam shot of you flicking a cigarette onto their corpse.
Fyi I Am A Spy: Backstab a Medic who has healed you in the last 5 seconds.
High Value Target: Backstab an enemy who is dominating 3 or more of your teammates.
Identity Theft: Backstab the enemy that you're currently disguised as.
Insurance Fraud: Kill an enemy while you're being healed by an enemy Medic.
Is It Safe?: Backstab 50 enemies who are capturing control points.
Joint Operation: Sap an enemy sentry gun within 3 seconds of a teammate sapping another.
May I Cut In?: Backstab an enemy and the Medic healing him within 10 seconds of each other.
On Her Majesty's Secret Surface: Start capping a capture point within a second of it becoming available.
Point Breaker: Kill 15 enemies who are standing on a control point they own.
Sap Auteur: Destroy 1000 Engineer buildings with sappers.
Sapsucker: Sap an enemy building, then backstab the Engineer who built it within 5 seconds.
Skullpluggery: Headshot 20 Snipers with the Ambassador.
Slash And Burn: Backstab an enemy, who then switches to Pyro before they respawn.
Sleeper Agent: Kill an enemy who triggered your feign death in the last 20 seconds.
Spies Like Us: While cloaked, bump into an enemy cloaked Spy.
Spymaster: Backstab 1000 enemies.
The Man From P.u.n.c.t.u.r.e.: Stab an enemy while fencing.
The Man With The Broken Guns: Backstab an Engineer, then sap 3 of his buildings within 10 seconds.
The Melbourne Supremacy: Dominate a Sniper.
Triplecrossed: Backstab 3 Snipers in a single life.
Wetwork: Stab an enemy to death while under the influence of Jarate.
Who's Your Daddy?: Headshot 3 Scouts with the Ambassador.
You Only Shiv Thrice: Backstab 3 enemies within 10 seconds.

Soldier Achievements - Soldier Update
These achievements can only be earned while playing the Soldier class

Ain't Got Time to Bleed: Kill 3 players with the Equalizer in a single life without being healed.
Backdraft Dodger: Kill a Pyro who has airblasted one of your rockets in the last 10 seconds.
Banner of Brothers: Buff 5 steam friends at once with the Buff Banner.
Bomb Squaddie: Destroy 10 sticky bombs with the shotgun in a single life.
Brothers in Harms: Kill 10 enemies while assisting or being assisted by another Soldier.
Crockets Are Such B.S.: Shoot two non-boosted crit rockets in a row.
Death from Above: Rocket jump and kill 2 enemies before you land.
Death From Below: Kill 10 opponents who are airborne with the Direct Hit.
Dominator: Get 3 dominations in a single life.
Duty Bound: While rocket jumping kill an enemy with the Equalizer before you land.
Engineer to Eternity: Kill an Engineer as he repairs his sentry gun while it's under enemy fire.
For Whom the Shell Trolls: Bounce an opponent into the air with a rocket and then kill them with the shotgun before they land.
Frags of our Fathers: Gib 1000 people.
Geneva Contravention: Kill 5 defenseless players after a single match has ended.
Gore-a! Gore-a! Gore-a!: Provide the enemy with a freezecam of you taunting over 3 of their body parts.
Guns of the Navar0wned: Kill 5 Engineer sentries while you are standing outside of their range.
Hamburger Hill: Defend a cap point 30 times.
Medals of Honor: Finish a round as an MVP 10 times.
Mutually Assured Destruction: Kill an enemy sniper with a rocket after he kills you.
Near Death Experience: Kill 20 enemies with your Equalizer while you have less than 25 health.
Out, Damned Scot!: Kill 500 enemy Demomen
Ride of the Valkartie: Ride the cart for 30 seconds.
S*M*A*S*H: Assist a Medic in exploding 5 enemies with a single ÜberCharge.
Screamin' Eagle: Kill 20 enemies from above.
Semper Fry: Kill 20 enemies while you are on fire.
Spray of Defeat: Use a grenade to gib a player.
The Boostie Boys: Buff 15 teammates with the Buff Banner in a single life.
The Longest Daze: Kill 5 stunned players.
Trench Warfare: Kill your nemesis with a shovel.
Tri-Splatteral Damage: Kill 3 enemies with a single critical rocket.
War Crime and Punishment: In a single life, kill 3 enemies who have damaged a Medic that is healing you.
War Crime Spybunal: Kill a Spy who just backstabbed a teammate.
Where Eagles Dare: Get the highest possible rocket jump using jump and crouch.
Wings of Glory: Kill an enemy soldier while both you and the target are airborne.
Worth a Thousand Words: Provide the enemy with a freezecam of your 21 gun salute.

Demo Achievements - Demo Update
These achievements can only be earned while playing the Demo class

Beat Me Up, Scotty: Use a full charge critical swing with the Eyelander to kill 5 enemy players.
Blind Fire: Kill an Engineer building that you can't see with a direct hit from your Grenade Launcher.
Bloody Merry: Provide an enemy player with a freeze cam of your smiling face.
Brainspotting: Decapitate 50 enemy players.
Bravehurt: Kill 200 players defending a capture point or cart.
Caber Toss: Bounce an enemy into the air and kill them before they land.
Cry Some Moor!: Destroy 50 buildings.
Double Mauled Scotch: Kill 2 people in a single sticky jump.
Glasg0wned: Kill 25 Scouts and Pyros with the Grenade Launcher.
He Who Celt It: Use the Sticky Launcher to kill an enemy player via environmental damage.
Highland Fling: Sticky jump a really long way...
Kilt in Action: Kill 500 enemy Soldiers.
Laddy Macdeth: Kill 50 enemies with direct hits from the Grenade Launcher.
Left 4 Heads: Decapitate 4 players with only 10 seconds between each kill.
Loch Ness Bombster: Kill an enemy player with sticky bombs within 5 seconds of them teleporting.
Pipebagger: Kill at least three players with a single detonation of sticky bombs.
Robbed Royal: Destroy 100 sticky bombs with the Scottish Resistance.
Scotch Guard: Kill 3 enemies capping or pushing a cart in a single Stickybomb detonation 3 separate times.
Scotch Tap: Glory in the slaughter of your enemies using the Eyelander.
Second Eye: Provide an enemy player with a freeze cam of you shaking your rump.
Shorn Connery: Decapitate a cloaked Spy.
Slammy Slayvis Woundya: Decapitate an enemy Soldier who is brandishing the Equalizer.
Something Stickied This Way Comes: Kill 30 players with air burst sticky bombs.
Spynal Tap: Kill 20 spies within 5 seconds of them sapping a friendly building.
Sticky Thump: Using the Scottish Resistance, kill 3 players in separate explosions without placing new sticky bombs.
Tam O'Shatter: Destroy 5 enemy Engineer buildings during a single ÜberCharge from a Medic.
Tartan Spartan: Do 1 million points of total blast damage.
The Argyle Sap: Blow up an Engineer, his sentry gun, and his dispenser with a single Sticky bomb detonation.
The High Road: Sticky jump onto a cap point and capture it.
The Scottish Play: Get a melee kill while sticky jumping.
The Stickening: Kill 5 Heavies from full health with a single sticky bomb detonation.
The Targe Charge: Charge and kill someone with your shield bash.
There Can Be Only One: Decapitate your nemesis.
Three Times a Laddy: Dominate three Engineers.
Well Plaid!: Kill 10 enemies while assisting or being assisted by another Demoman.

Halloween Achievements
These achievements are only available to be unlock between October 29th-November 2nd.